Federal report sees human-caused changes to California’s climate

During the state’s recent drought, the Almaden Reservoir in San Jose went dry, revealing in 2014 a car that had been dumped in the lake. The federal government’s latest National Climate Assessment says California can expect more extreme swings between droughts
and floods
unless greenhouse gas emissions are cut sharply. less

During the state’s recent drought, the Almaden Reservoir in San Jose went dry, revealing in 2014 a car that had been dumped in the lake. The federal government’s latest National Climate Assessment says ... more

Photo: Michael Short, The Chronicle

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Michael Murawski naps in Yerba Buena Gardens in San Francisco on Friday, July 28, 2017.

Michael Murawski naps in Yerba Buena Gardens in San Francisco on Friday, July 28, 2017.

Photo: Nicole Boliaux, The Chronicle

Federal report sees human-caused changes to California’s climate

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The changes to California’s climate since 1980 — higher temperatures, with more extreme swings between droughts and floods — are caused directly by human activity and will accelerate rapidly unless greenhouse gas emissions are cut sharply, according to a new federal climate report that is awaiting action by the Trump administration.

The fourth National Climate Assessment, a federal synthesis of climate science required every four years by law, says temperatures have risen rapidly since the last report was published in 2014. After setting a record that year, global temperatures shot to a new record by a wide margin in 2015, the report says, followed by another record last year.

The report, produced by 13 federal agencies and approved by the National Academy of Sciences, is unequivocal in ascribing the warming to human activity, a finding that the Trump administration and many Republicans in Congress have disputed.

“One of the clearest signals that is summarized in this report is that California is already a warmer place than it used to be,” said Daniel Swain, a climate scientist at UCLA whose work is cited in the report. “That’s not a future prediction anymore. It’s the reality we’re living, and the warming we’re seeing so far pales in comparison to what we’re likely to see in the future on our current carbon emissions trajectory.”

Although the report does not directly attribute the state’s recent five-year drought to climate change, California scientists whose work the report cites said hotter temperatures vastly increase the chances of drought by drying soils and plants, as well as causing more precipitation to fall as rain instead of snow, and melting the Sierra snowpack earlier.

“It’s very clear that temperatures in California are increasing the risk of severe drought,” said Noah Diffenbaugh, a climate scientist at Stanford University. “During this recent drought, we had the warmest years on record, the warmest winters on record, the most severe combination of low precipitation and high temperature on record, the lowest snowpack on record, the most drought indicators on record. These are all physically very closely linked with high temperatures.”

The draft report was published by the New York Times on Tuesday from publicly available information. The Times followed with publication of the final report. In the past, the reports have been published by the 13 agencies at globalchange.gov.

The report predicts that U.S. temperatures will rise at least 2.5 degrees Fahrenheit over the next few decades, with cities experiencing much hotter temperatures. The East will get wetter, and the Southwest drier. The hydrology of the Colorado River Basin, on which the West’s big cities such as Phoenix, Los Angeles and Las Vegas depend, has already undergone “profound change,” the report says.

Atmospheric rivers that bring extreme precipitation to the West Coast, like the ones last winter that ended California’s drought, are projected to increase in frequency, but will fall more as rain than snow, the report says. That would disrupt California’s highly engineered plumbing system, which relies on the Sierra snowpack to store water for the dry summers.

Forest fires have already increased sharply in the West, the report says, and will increase more as the region continues to dry, bringing major ecosystem changes.

The report says a sharp reduction in carbon emissions could limit the global average increase in temperatures to 3.6 degrees Fahrenheit by the end of the century. The climate has warmed 1.2 degrees Fahrenheit since 1986, the report says.

Without a major reduction in emissions, warming may rise 9 degrees Fahrenheit by the end of the century, a level that would make parts of the world uninhabitable and cause an extraordinary rise in sea level, the report says.

The report is “without doubt alarming,” said Peter Gleick, chief scientist and co-founder of the Pacific Institute, an Oakland think tank that researches water issues. Gleick predicted the effects of rising heat on California’s water supply 30 years ago.

“This has been alarming for a long time,” Gleick said. “Humans are fundamentally changing the climate of the planet. It should be alarming to everyone, because the impacts are going to get worse and worse if the politicians don’t act on what the science is telling them.”

Richard Alley, a geoscientist at Penn State University, called it “good, solid, well-accepted, carefully vetted science. ... There’s nothing over the top, nothing outside the pale. It’s what we knew, written up.”

Alley said the changes in the climate that have already occurred are small compared with what will happen if emissions are not reduced sharply.

“At some point, flooding goes from being a nuisance to building a new levee or retreating from the shore,” he said. “We could make a world in which it starts to become fatal in some places to be outside, and well before that, it gets hard to work outside for a lot of people in a lot of places.”

Alley said the cost of reducing emissions now pales beside the cost of dealing with future climate change.

“As the change gets bigger, the costs go up faster than the temperature,” he said. “If we stabilized concentrations in the atmosphere today — we can’t, but if we did — warming would continue, sea level would continue to rise, so we have to deal with some change.”

Right now, he said, the decision is “whether the change becomes a lot bigger.”